linux-kernel - Re: [PATCH] fork: Improve error message for corrupted page tables

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Date:   Wed, 31 Jul 2019 15:27:53 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        dave.hansen@...el.com, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] fork: Improve error message for corrupted page tables

On Tue, 30 Jul 2019 15:18:20 -0700 Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com> wrote:

> When a user process exits, the kernel cleans up the mm_struct of the user
> process and during cleanup, check_mm() checks the page tables of the user
> process for corruption (E.g: unexpected page flags set/cleared). For
> corrupted page tables, the error message printed by check_mm() isn't very
> clear as it prints the loop index instead of page table type (E.g: Resident
> file mapping pages vs Resident shared memory pages). Hence, improve the
> error message so that it's more informative.
> 
> Without patch:
> --------------
> [  204.836425] mm/pgtable-generic.c:29: bad p4d 0000000089eb4e92(800000025f941467)
> [  204.836544] BUG: Bad rss-counter state mm:00000000f75895ea idx:0 val:2
> [  204.836615] BUG: Bad rss-counter state mm:00000000f75895ea idx:1 val:5
> [  204.836685] BUG: non-zero pgtables_bytes on freeing mm: 20480
> 
> With patch:
> -----------
> [   69.815453] mm/pgtable-generic.c:29: bad p4d 0000000084653642(800000025ca37467)
> [   69.815872] BUG: Bad rss-counter state mm:00000000014a6c03 type:MM_FILEPAGES val:2
> [   69.815962] BUG: Bad rss-counter state mm:00000000014a6c03 type:MM_ANONPAGES val:5
> [   69.816050] BUG: non-zero pgtables_bytes on freeing mm: 20480

Seems useful.

> --- a/include/linux/mm_types_task.h
> +++ b/include/linux/mm_types_task.h
> @@ -44,6 +44,13 @@ enum {
>  	NR_MM_COUNTERS
>  };
>  
> +static const char * const resident_page_types[NR_MM_COUNTERS] = {
> +	"MM_FILEPAGES",
> +	"MM_ANONPAGES",
> +	"MM_SWAPENTS",
> +	"MM_SHMEMPAGES",
> +};

But please let's not put this in a header file.  We're asking the
compiler to put a copy of all of this into every compilation unit which
includes the header.  Presumably the compiler is smart enough not to
do that, but it's not good practice.